1. Field of the Invention
This invention pertains to measuring apparatus, and more particularly to apparatus for measuring gas characteristics.
2. Description of the Prior Art
The present invention is a further development of the subject matter of our U.S. Pat. Nos. 4,101,404 and 4,186,072.
It is well known to measure the oxygen content of gases by using a probe comprising a solid electrolyte which is in contact on one side with a known reference gas and on the opposite side with the gas whose oxygen content is to be measured. In such a probe, a voltage is generated between the two sides of the electrolyte. The magnitude of the voltage is dependent upon the temperature of the electrolyte and on the log of the ratio of the oxygen partial pressures on the opposing side of the electrolyte. This principle has been used in the past to measure the oxygen partial pressure of a hot furnace gas with various types of oxygen sensors, such as those disclosed in U.S. Pat. Nos. 3,454,486; 3,546,086; 3,597,345 and British Pat. No. 1,296,995.
The major components of all oxygen sensors are the electrolyte, the electrodes, i.e., anode, cathode, and the electrical lead wires which make electrical contact with the electrodes. The selection of a proper electrode material generally requires the satisfaction of chemical, electro-chemical, mechanical, and economical criteria. Platinum and other noble metals often meet the performance criteria, but they are very expensive. The specific electrode material and design are usually dependent on the particular environment to which it is subjected.
Commercial solid electrolyte oxygen sensors have been used in a wide variety of applications, such as motor vehicle exhaust, flue gas, molten copper or steel, and metal heat treating. As an example of the latter application, oxygen sensors are used for controlling the carbon potential in carburizing atmospheres. Control of Surface Carbon Content, Metals Handbood, Vol. 4, p. 417-431, 9th Edition, 1981. In addition to the aforementioned patents, oxygen sensors are disclosed in U.S. Pat. No. 4,193,857, German Pat. No. 2,401,134, and Australian Provisional Specification 47,828/79.
Although oxygen sensors have been used for several years and the principle of operation in those applications is quite simple, two major problems remain. The first is that the accuracy and repeatability of some commercial oxygen sensors are not high enough for heat treating applications requiring close carbon potential control. That is, the relationship between voltage, temperature, and percent carbon sometimes varies from sensor to sensor or with sensor usage. Carburizing and Carbonitriding, page 81, American Society For Metals, Metals Park, Ohio, 1977.
To accurately determine the oxygen partial pressure of the gaseous atmosphere in a heat treating furnace, the electrode in the furnace atmosphere (the anode) should maintain thermodynamic equilibrium between the oxygen activity within the atmosphere as a whole and the oxygen activity at the surface of the electrolyte. This condition is not always met in prior oxygen sensors.
The second major problem is that prior oxygen sensors have useful lives which are frequently limited because of anode or lead wire failures. Most failures are caused by chemical reactions between the electrodes and/or the lead wire and impurities in the furnace atmosphere. Even anodes made of noble metals such as platinum are susceptible to attack from contaminants such as zinc, silica, and sulphur.
Because platinum and other noble metals are expensive, and because anodes made from noble metals frequently have short useful lives, attention has been directed to other materials which might be suitable for anodes. Pure nickel, for example, has been used for anodes in fuel cells. However, although nickel possesses high temperature properties desirable in gas probes, it has been discovered that in heat treat atmospheres where the gases are not in equilibrium nickel acts as a catalyst which alters the gas composition locally which results in a nonrepresentative voltage output. Specifically, in heat treating atmospheres it has been found by our tests that the presence of nickel increases the rate of reaction of methane with water or carbon dioxide adjacent the anode surface. The altered gas composition generates a larger and non-representative voltage.
Other disadvantages of pure nickel electrodes are the formation of nickel oxide on the surface of the electrodes when exposed to an oxidizing atmosphere, and the formation of a low melting temperature nickel-sulphur eutectic when exposed to gases containing sulphur.
U.S. Pat. No. 4,193,857 indicates that certain nickel alloys may be useful as electrodes. However, the alloys disclosed in that patent are not entirely satisfactory either because the nickel content of the alloys disclosed is so high as to cause the aforementioned undesirable catalytic reactions, or because certain important alloying elements are lacking which render the electrode unable to meet all the design criteria. Heretofore it was not recognized in the art that metal electrodes catalyze reactions of the common enriching gases used for carburizing steel parts and thereby change the composition of the gas locally adjacent to the probe being measured and hence alter the output of the sensor and obtain distorted measurements by providing higher voltage readings. The common enriching gases include methane, propane and butane. The reason the nickel electrode sensor gave a higher emf reading was the catalytic reaction of the nickel electrode on the following reactions for example: EQU CH.sub.4 +H.sub.2 O=CO+3H.sub.2 EQU CH.sub.4 +CO.sub.2 =2CO+2H.sub.2
(i.e., the rate of the reaction of CH.sub.4 with H.sub.2 O or CO.sub.2 was increased and thus reduced the amount of H.sub.2 O and CO.sub.2 in the gas phase that was adjacent to the surface of the nickel electrode). This effect produced a lower oxygen potential at the electrode and therefore produced a larger emf.
Our tests have revealed that platinum anodes have also produced the above described catalytic reaction. Workers in the art have not recognized this catalytic reaction and the need to minimize the locally altered gas composition with adequate ventilation as subsequently described in detail. Several prior art probe constructions obstruct gas flow adjacent the anode with a packing or fill in the gas inlet passages to filter or prevent particulate debris from entering the probe. This teaching is contrary to the objectives of this invention and indicates that other workers in the art have not recognized the catalytic reaction described above and the resulting effect on the accuracy of a probe. An example of packing in the gas inlet is a probe sold by Barber-Colman which appears to be made in accordance with U.S. Pat. No. 4,193,857.